As a denitrification method in the wastewater treatment method, an activated sludge recycle process in which sludge is circulated between an anoxic tank in which free oxygen is not present and an aeration tank, which releases air into the sludge from an air diffuser, ammoniac nitrogen is oxidized to nitrate-nitrogen in the aeration tank, and the nitrate-nitrogen is reduced in the anoxic tank to be discharged out of a system as nitrogen gas has been conventionally widely performed. However, according to this method, although nitrogen may be efficiently removed, phosphorous is not sufficiently removed. This is because an anaerobic degree in the anoxic tank is not sufficiently raised due to dissolved oxygen, the nitrate-nitrogen and the nitrite-nitrogen included in circulated water from an aerobic tank, and phosphorous release from phosphorous accumulating bacteria is not sufficiently performed.
Therefore, when it is required to simultaneously perform denitrification and dephosphorization, a so-called A2/O method in which the denitrification and the dephosphorization are biochemically performed by arranging an obligatory anaerobic tank, which does not include the free oxygen and bound oxygen such as a nitric acid and a nitrous acid, before the anoxic tank (denitrification tank) of the above-described activated sludge recycle process is used. In the obligatory anaerobic tank, a polyphosphoric acid by the polyphosphate accumulating bacteria is hydrolyzed to dissolve out the phosphorous and introduces a biochemical oxygen demand (hereinafter, referred to as a BOD) component into a bacteria cell. It is said that, in order to maintain an obligatory anaerobic condition, the BOD is required to be 50 mg/L or higher.
However, there is a problem in the wastewater treatment method according to the A2/O method that an obligatory anaerobic tank should be additionally provided and a large device installation area is required as compared to the activated sludge recycle method.
Therefore, for example, WO03/101896 (Patent Document 1) discloses the wastewater treatment method capable of removing the nitrogen and the phosphorous only by the two treatment tanks, which are the anoxic tank and the aeration tank, without the aggregating agent. The wastewater treatment method is the activate sludge treatment apparatus for performing the so-called activated sludge recycle method, configured to circulate the sludge between the anoxic tank and the aeration tank to biochemically treat the wastewater, and this is configured to take out the sludge being the circulated fluid below the air diffuser disposed on the lowest position in the aeration tank when sending the sludge being the circulated fluid from the aeration tank to the anoxic tank and sets the DOC (hereinafter, referred to as the DOC) at the site to take out the sludge to 0.5 mg/L or less, and sets the DOC at the site at which the sludge sent from the aeration tank enters the anoxic tank to preferably 0.2 mg/L or less. It is said that, by doing so, both of the nitrogen and the phosphorous may be removed by using only the two treatment tanks, which are the anoxic tank and the aeration tank, without using the aggregation agent and the like.
In the meantime, in the above-described aeration tank in this type of activated sludge treatment, as disclosed in the Patent Document 1, the membrane filter unit provided with a hollow fiber membrane module assembled by arranging in parallel a plurality of sheet-like hollow fiber membrane elements in which a great number of porous hollow fibers are used so as to be parallel, and the air diffuser, which is a minute bubble generating unit, arranged below the hollow fiber membrane module for releasing the air toward the hollow fiber membrane module is often used for a reason that biochemical treatment efficiency and filtration efficiency of the activated sludge are high.
A predetermined amount of air is released from the air diffuser into the sludge as minute bubbles, this becomes a gas-liquid mixed flow while moving upward in the sludge and moves upward, flows upward in a gap in the hollow fiber membrane module, and the flow, which passes through the hollow fiber membrane module, flows down along an outer side of the membrane filter unit to form a gas-liquid mixed swirling flow. While the gas-liquid mixed swirling flow vertically swirls, the oxygen in the air becomes the free oxygen and is dissolved in the sludge, decomposes or nitrifies the organic matter and the like, and/or, introduces the phosphorous into the polyphosphate accumulating bacteria and allows the same to grow to allow the sludge to grow, and also, as disclosed in Japanese Patent Laid-Open Publication No. 2000-51672 (Patent Document 2) and Japanese Patent Laid-Open Publication No. 2000-84553 (Patent Document 3), for example, a contaminant attached to the hollow fiber membrane element is exfoliated from the same to clean the same by so-called air scrubbing for allowing the gas-liquid mixed flow to act on the component hollow fiber membrane of the hollow fiber membrane module to oscillate. At the same time, the sludge is sucked by a suction pump and the solid-liquid separation is performed through the porous hollow fiber membrane, and treated water is sent to an external treated water tank through a hollow portion of the hollow fiber membrane.    Patent Document 1: WO03/101896    Patent Document 2: Japanese Patent Laid-Open Publication No. 2000-51672    Patent Document 3: Japanese Patent Laid-Open Publication No. 2000-84553